in a race u drove 1st lap with 40kmph and in the second lap at what speed u must drive so that ur average speed must be 80kmph.

cannot be determined
that is average speed= (2*speed1*speed2)/(speed1+speed2)
but here.......................................................................
80=2*40*x/(40+x)
cannot be solved
or in otherwise x is infinity

You have to draw 3 concentric circles with a line passing thru their center without lifting hand.

yes its possible...
1. draw the diameter of first circle
2. draw the circle without lifting hands until it reaches the start point on the diameter
3. extend the diameter
4. draw circle as step 2
5. repeat 3 and 5
3.

There are 9 coins. 8 are of 1 gm and 1 is of 2 grams. How will you find out the heavier coin in minimum number of weighing and how many weighing it will need?

divide 9 coins into three groups...(a group of three)
select any two group and weigh them
if they are equal then the odd coin is in the other group . if the weights are not equal select the heavier grou and divide into three .
select any two and weigh them ..if they are equal the odd coin is the other one which is outside the weighing pan . or select the heavier one from pan.
thus a total of "two" iteration are needed.

How will you recognize the magnet & magnetic material & non-magnetic material?

The magnet has maximum magnetic strength near the ends as compared to middle portion.
Mark 3 items as A, B and C.
Bring End of material A near the middle portion of B and C one by one.
If B is attracted towards A, then B is magnetic material and A is magnet.
C is not attracted, then C is non magnetic.

If none of B and C is attracted towards A, then Bring A closer to end of B and C one by one.
If B attracts A, then B is magnet and C is non magnetic material and A is magnetic material.

If still no one attracts A or attracted by A , then A is non magnetic material.

Repulsion is the sure test of magnetism. Thus, two magnets repel each other. A magnet can thus be identified. Non-maghnetic materials won't be attracted. Magnetic materials will always be attracted by a magnet.